In this new age of concussion safety, it doesn’t usually happen this way. But all three Rams starters who went through the concussion protocol this week have been cleared to play Sunday in San Francisco.Running back Zac Stacy, left guard Chris Williams, and cornerback Trumaine Johnson all were listed as having full participation in practice Friday and are listed as probable for the 49ers’ game. By NFL definition, that means they have at least a 75 percent chance of playing.

“I was good the whole time,” Stacy said after Friday’s practice. “It was just a precaution thing. I’m ready to go. I’ve had a great week of practice.”

With 12 carries for 87 yards in the opening two quarters vs. Chicago, Stacy was well on his way to a career rushing day when taken to the locker room just before halftime.

“It was one of those deals where I kind of just took a shot,” said Stacy, the rookie from Vanderbilt. “I kept playing with it. I was fine.”

But when the team’s medical and athletic training staff keep you off the field, there’s nothing you can do about it.

“It was hard,” Stacy said. “I was disappointed because I’m always trying to help my team.”

But he added: “Safety’s always still the first issue. You’ve just got to do what the doctors say. I’m just glad I was able to practice this week. Get the looks. Get the reps. So I’m good to go.”

Stacy was pretty much a bystander the first time the Rams met the 49ers, on Sept. 26 in a Thursday night game at the Edward Jones Dome. He was in for only one snap on offense and didn’t touch the football in a 35-11 loss. He has been the Rams’ starter in every game since then, gaining 620 yards on 140 carries over seven starts.

“San Fran’s got a heck of a defense,” Stacy said. “Obviously the core of their defense is the linebackers and the front seven with NaVorro Bowman and Patrick Willis. It’s gonna be a challenge for us.”

Helping to open holes for Stacy over those seven starts was Williams. In fact, Williams was blocking on a 16-yard run by Stacy when Williams’ helmet struck the hip of a Bears defensive tackle. The last thing Williams wanted to do was leave in the middle of his first game against his former team, but that’s what happened.

“At the time it’s frustrating because you want to play,” Williams said. “And we were playing Chicago and that kind of thing. But (the protocol) is implemented to protect us. So you really can’t argue with that.

“When they say you’re done, that’s definitely a bummer. But we won, so good tradeoff I guess.”

Shelley Smith, who shared time with Williams against Jacksonville, Houston, and Carolina, took over at left guard for the rest of the Chicago game. Williams sounded confident about playing against San Francisco, although he didn’t express it with the certainty of Stacy.